The present invention pertains to an electrochemical secondary element having at least one electrode with an active material comprised of a polymeric compound of polyconjugated structure, which can be doped with cations or anions and which can be formed as a thin, film-like layer.
Polymers with a polyconjugated structure of the main chain are basically insulators. However, it has been shown (see, Heeger and MacDiarmid, J. Chem. Soc. Chem. Comm., 1977, page 578) that the organic polymer polyacetylene in the form of a thin film acquires considerable conductivity by reacting with oxidizers such as iodine, bromine and arsenic pentafluoride. This increased conductivity is several orders of magnitude greater than the conductivity of the compound in its initial state.
The result of the oxidation (removal of y electrons) of the polyacetylene is electron-deficient conducting polymer chains (CH.sup.+y).sub.x. For stabilization, a corresponding amount of a reduced oxidizing agent A is added, so that the oxidized polyacetylene has the composition (CH.sup.+y A.sup.-.sub.y).sub.x. If, for example, iodine is used as the oxidizing agent, then I.sub.3.sup.- ions are accepted as stabilizing opposing ions. Heeger and MacDiarmid also showed that the oxidation of polyacetylene can be accomplished not only by the above-described chemical means, in which the increase in conductivity comes about as a result of a charge transfer between the polymer and the oxidizing agent, but also by electrochemical means. Moreover, polyacetylene can not only be oxidized in an electrochemical cell, but also reduced, which leads to increased conductivity resulting from an excess of electrons along the polymer chains.
For this purpose, two polymer foils are provided with metallic conductors (e.g., platinum) and immersed in a solution of a conducting salt, e.g., lithium perchlorate (LiClO.sub.4) in propylene carbonate (PC). When a voltage is applied to this arrangement, electrons are removed from one of the polymers and supplied to the other polymer, i.e., one of the polymers is oxidized and experiences an increase in conductivity corresponding to the charge conversion, while the other polymer is reduced, with a corresponding increase in conductivity. To stabilize the radical cations and radical anions that are formed, the polymers then accept a corresponding number of anions and cations (in this example, ClO.sub.4.sup.- and Li.sup.+ ions).
Polymeric compounds which have been converted to the conductive state by electrochemical means are interesting as electrode materials, especially for secondary batteries, because their electrochemical oxidation and reduction can be reversibly performed. EP-OS No. 111,237, for example, specifies a series of known electrochemical systems based on such polymers.
The preparation of the polymer material itself differs from substance to substance. A polyacetylene film, for example, can be prepared by bringing the corresponding monomer into contact with a concentrated solution of a Ziegler catalyst in an inert solvent. When acetylene is blown onto a stationary surface of the catalyst solution, a thin layer of polyacetylene is formed which floats on the surface and which can be removed from the reaction medium. In a variation of this method, an inert carrier (e.g., a glass plate) can be wetted with the catalyst solution to obtain an easily removable polymer layer. Electropolymerization is another method which can be used for the direct production of electron-conducting polypyrrole, polythiophene or polyaniline (e.g., DE-OS No. 3,402,133). In this case, the polymer is deposited by anodic current on an electrically conducting surface from an electrolytic bath including an aqueous or organic solution of the monomer containing a salt with anions suitable for the oxidation.
However, in performing discharge experiments on such electrodes, it was found that at a constant discharge voltage, the duration of the discharge increased more than proportionally with the thickness of the polymer films accumulated during the electrochemical oxidation. Therefore, it was not possible, in an acceptable amount of time, to achieve a charge conversion in proportion to the polymerization time, and accordingly, to the polymer film thickness.